A known belt type continuously variable transmission controller is configured to perform a belt slip control in which an actual secondary hydraulic pressure is reduced from one during a normal control to slip a belt wound around pulleys at a predetermined slip rate by controlling the actual secondary hydraulic pressure on the basis of a multiplier of an oscillation component included in the actual secondary hydraulic pressure and an oscillation component included in an actual gear ratio. This eliminates the necessity for directly detecting the belt slip rate and thereby facilitates the belt slip control (see Patent Document 1, for example).